Quantcast is a company that provides demographic data for websites. They can do it via a javascript tag the site owner embeds in the site (which The Forge has not done, AFAIK) or they can estimate demographics.

Odd curve--I can't think of any reason for traffic to have fallen so precipitously from the New Year high to a March low and then come back up again. I plugged in my own domain (MJYoung.net) and got something very opposite, spiking in March. I also plugged in rpg.net, but they apparently have arranged to be "quantified" so their stats are considerably more detailed; they show a more level curve.

Interesting data, but I want to know where the Forge reveals peoples children, ethnicity and income?

I suspect it's done in part by using exit information from sites that are tracked in more detail. That is, if someone goes to RPGnet, a lot more data is gathered about them (because the site registered to be quantified, and so presumably has installed reporting software); if they click a link from RPGnet to here, that information is used to identify some quantity of users here, extrapolated to cover the users who come from sites where there is less tracking information. RPGnet has member profiles, so that might be the basis for determining gender and other information, although there are probably other details as well.